1. Field of the Invention
This invention concerns knife cylinders for cross-cutting or cross-perforating weblike material such as paper, plastic foil, metal foil or fabrics. The knife cylinders are formed with at least one groove for receiving a knife positioned essentially parallel to the axis of rotation of the cylinder and include pressure points distributed over the length of the knife for bracing the knife within the cylinder.
2. Description of the Prior Art
Knife cylinders of the type to which this invention is directed are used in machines for producing cuts in weblike materials. These cuts can be produced with knives having a discontinuous edge for producing a perforation or having a continuous edge for cutting off sheets of a particular length from an endless web. The cut lines in this instance usually run transverse to the direction of travel of the web, but they can also be positioned obliquely to the direction of travel of the web.
The knives are customarily set in a rotating knife cylinder such that knives which have become dull can be replaced without having to remove the entire cylinder from the working machine. At least one knife is fastened to each knife cylinder, although, several knives can be distributed over the circumference of the knife cylinder in order to obtain cuts which have a closer spacing to each other than that corresponding to the diameter of the knife cylinder.
For the knives to be able to function in the desired manner, they must be set in relation to the web to appropriately be worked or in relation to a pressure cylinder cooperating with the knife cylinder. The knives must therefore be fastened in a non-positive manner to the knife cylinders. The knives are customarily braced in relation to the knife cylinder with the aid of wedges such as disclosed in U.S. Pat. No. 4,187,753. Here, several screws are associated with each knife and are distributed over the length of the knife. However, even if only one single dull knife is to be replaced by a sharp knife, a plurality of screws must be loosened and then retightened. This replacement requires considerable time and thus considerably delays production during use of the working machine.
As an improvement, published German Pat. application DE-OS No. 2,355,290 discloses a rapid tensioning system which utilizes a wedge and cylindrical tension pieces. Several cylindrical tension rollers are associated with a knife, whereby the knife is braced against the knife cylinder by a reciprocal bracing of the rollers. The knife can be loosened along its entire length after only a single part has been loosened and can thus be rapidly removed from the machine. The knife can also be rapidly braced against a groove in the knife cylinder.
If several knives are distributed around the circumference of a knife cylinder, each knife must be set in relation to the associated pressure cylinder cooperating with the knife cylinder. This is generally accomplished by first pretensioning all knives with a relatively slight pressure. Then, the knife cylinder is run opposite the pressure cylinder while the web is passing therethrough, whereby the knives are shifted by minute amounts, generally in a radial direction. The knives are not tightened with full force until this adjustment is completed and must be initially fastened in a non-positive manner due to the necessary adjusting.
Since the knives should not shift when the machine is in operation, a large force is required to tighten the knives firmly in place. This force is so great that all machine parts associated with the tightening procedure can be deformed, at least over a period of time. For example, the device disclosed in the aforementioned DE-OS No. 2,355,290 is susceptible to such deformation.
The tension rollers of published German patent application DE-OS No. 2,355,290 are deformed due to the large force produced by the tensioning device. The tensioning device thus becomes loose during the operation of the knife cylinder due to its construction thereby causing it to undesireably shift. The loosened knife thus produces either an unesired cut or not cut at all, or the knife becomes dislodged during the operation of the machine.
Published German patent application DE-OS No. 1,939,358 discloses a device including a hydraulic pressure ensuring system for fastening printing forms on the printing cylinders of printing machines. The pressure generator associated with the system is fastened to the printing cylinder. However, the force required to tension printing forms is considerably less than that required to tension knives. Thus, the tensioning system of DE-OS No. 1,939,358 is ineffective for the tensioning of knives.
Published German Pat. application DE-OS No. 2,244,077 discloses a device for clamping machining tools in which the knives clamped in a knife driving shaft can also be tensioned hydraulically. In this instance, however, the knives are clamped under quite different conditions compared to that required for knife cylinders for working webs, e.g., for cross-cutting or cross-perforating. There is also a different geometry for tensioning the knives, which results in a different clamping force. This geometry is based on the different functions required of these knives.